Factors for determining the information reading accuracy in a CD player or an LVD player include skew (or tilt). Such skew is caused by warp of a disk per se, and in addition to such warp, particularly in the case of an LVD of a large diameter, a portion of the LVD projecting from a turntable may hang down, thus causing skew. This is because a turntable is restricted in size as much as possible so as to reduce the inertia of a rotary system and therefore an outer circumferential portion of an LVD projected over the turntable becomes inclined. In such away, the skew causes a state where the orthogonal relation between the information recording surface of a disk and the optical axis of an optical pickup cannot be maintained. As a result, the reading accuracy of a reading apparatus is reduced because the reading apparatus is based on the assumption that there is an orthogonal (or fixed-angular) relation between the optical axis and the information recording surface. In order to solve such a problem, a tilt detector is used to correct the tilt of an optical pickup corresponding to the skew.
FIG. 3 shows an example of the conventional tilt detector.
In this example, a tilt detector B is attached on the outside of an optical integration circuit type pickup A.
In the optical integration circuit type pickup A, a semiconductor laser 2 is provided on an end of an optical integration circuit substrate 1 so that light injected from the semiconductor laser 2 is guided to a condensing grating coupler 3 through an optical waveguide 4 formed in the optical integration circuit substrate 1. The light is condensed by an objective lens function of the condensing grating coupler 3 so as to form a main beam spot 8 upon an information recording surface. The light reflected from the information recording surface is returned to the condensing grating coupler 3 again, is guided through the optical waveguide path 4, is led to a Foucault prism 6 by means of a beam splitter 5, and is then applied to light detection elements 7a-7d. The light detection elements 7a-7d produce output voltages, respectively, in accordance with the incident light beams so that a focus error signal, a tracking error signal, and a read signal are obtained through operations performed upon the respective output signals of the light detection elements 7a-7d by means of an operator (not shown). Known examples of such an optical integration circuit type pickup A include those described in "LIGHT INTEGRATION DISK PICKUP HEAD" (OPTRONICS, 1989, No. 2, pp 149-154) and Japanese Patent Unexamined Publication No. 61-296540, etc.
The optical integration circuit type pickup A as described above is provided with the tilt detector B at its condensing grating coupler side as shown in FIG. 3. In the tilt detector B, first and second light sensors 11 and 12 are disposed on a substrate 9 at a predetermined interval, and a light source 10 is disposed at an intermediate portion between the sensors 11 and 12. An information recording surface is irradiated with light beams emitted from the light source 10, so that first and second sub-beam spots 13 and 14 are formed on the information recording surface at positions separated from each other. The respective beams reflected from the first and second sub-beam spots 13 and 14 are returned to the first and second light sensors 11 and 12. The tilt is detected by judging the quantities of light beams returned to the first and second light sensors 11 and 12. That is, if the optical axis is perpendicular to a disk 16 as shown in FIG. 4, the output voltage of the first light sensor 11 is equal to that of the second light sensor 12. If a tilt has occurred, on the contrary, one of the reflection beams 17 comes away from the second light sensor 12 as shown in FIG. 5. Consequently, a difference is generated between the respective output voltages of the first and second light sensors 11 and 12. It is therefore possible to detect the tilt on the basis of a signal representing the difference of the output voltages.
The foregoing conventional reading apparatus is disadvantageous in that a detection error is apt to be generated because the first and second sub-beam spots 13 and 14 caused by the tilt detector B are apart from the main beam spot 8 caused by the optical integration circuit type pickup A. The apparatus is complicated and large in size because it is necessary to use parts for attaching the tilt detector B on the optical integration circuit type pickup A. The increase in size of the apparatus is opposed to the requirement for minimizing an apparatus as much as possible because use of the attachment parts causes increase of the number of parts.